Cell Cycle
The cell cycle, first described by Howard and Pelc in 1953, is a genetically controlled series of changes that occur in a newly formed cell, leading to its growth, content replication, and division to form two daughter cells. It consists of two major phases: a long non-dividing growth phase called the interphase (I-phase) and a short dividing phase called the mitotic phase (M-phase), each with substages.
- The sequence of the G1, S, G2 (interphase), and M phases (mitotic phase) is known as the cell cycle.
- Although the interphase is often referred to as the "resting stage," it is actually a period of intense cellular activity. Three important processes occur during this phase in preparation for cell division, making it the preparatory phase:
- Replication of DNA along with the synthesis of nuclear proteins such as histones.
- In animal cells, centrioles duplicate, with daughter centrioles growing from the parent centrioles positioned at right angles to each other.
- Synthesis of energy-rich compounds that provide energy for mitosis, and protein synthesis near the end of interphase.
Subphases of Interphase
Interphase is divided into three subphases:
G1 Phase (First Growth Phase)
It is also known as the post-mitotic gap phase. G1 is the longest phase of cell division, where various types of RNA (mRNA, tRNA, rRNA) and proteins are synthesized.
All cell organelles, including the endoplasmic reticulum, mitochondria, Golgi complex, and ribosomes, multiply during this phase. The duration of the G1 phase varies across different cell types and is shorter in rapidly dividing cells.
Cells in the G1 phase have three possible fates:
1. They can proceed with the cell cycle and enter the S phase.
2. They can pause the cell cycle and enter the quiescent G0 phase.
3. They can stop the cell cycle and undergo cell differentiation.
The decision to proceed with one of these options depends on the availability of mitogens and energy-rich compounds, a control point known as the "checkpoint."
S Phase
- During the S phase, DNA replication occurs, and histones are synthesized, leading to the formation of two chromatids for each chromosome. This results in each chromosome carrying a duplicate set of genes.
- A haploid cell (n) becomes diploid (2n), and a diploid cell (2n) becomes tetraploid (4n) by the end of the S phase. Additionally, damaged DNA undergoes repair during this phase.
G2 Phase (Second Growth Phase)
- The G2 phase, also known as the pre-mitotic gap phase, involves the cessation of DNA synthesis, while RNA and protein synthesis continue. Cell organelles multiply, and spindle formation begins.
- This phase lasts for 2-5 hours in most cells. Specific proteins produced during the G2 phase trigger chromosome condensation, marking the onset of mitosis.
G0 Phase (Quiescent Phase)
- The G0 phase is characterized by cells that do not proceed to the S phase after the G1 phase. Instead, they stop dividing and undergo differentiation. This occurs due to the lack of mitogens or energy-rich compounds.
- Cells in the G0 phase remain metabolically active, grow in size, and differentiate to perform specialized functions. Some cells, such as fibroblasts, remain undifferentiated and can resume cell division when required, such as during wound healing.
M Phase (Mitotic Phase)
The M phase, or Mitotic phase, is the stage of the cell cycle during which a cell divides to produce two genetically identical daughter cells. This phase involves both the division of the nucleus (mitosis) and the division of the cytoplasm (cytokinesis). The M phase is much shorter than interphase and is highly organized. It ensures the equal distribution of duplicated chromosomes to each daughter cell. The events of mitosis are subdivided into several stages: prophase, prometaphase, metaphase, anaphase, and telophase, followed by cytokinesis.
The process of cell division, which is similar in all living organisms, primarily involves changes in the nucleus. There are three types of cell division:
1. Amitosis (direct cell division)
2. Mitosis (indirect cell division)
3. Meiosis (reduction division)
